The present invention generally relates to the field of portable illumination devices for illuminating an ambient environment. More specifically, the present invention is directed to a hand-held laser flashlight which illuminates an ambient environment while minimizing the risk of causing irreversible eye damage.
Law enforcement, corrections, and military personnel face increasingly greater threats in their daily activities. Routine traffic stops can end in officers finding themselves in life-threatening situations. Domestic disputes and drug enforcement activities are amongst the most dangerous law enforcement personnel face. Also, increasingly law enforcement personnel face situations such as riots or unruly groups of individuals where certain lethal options cannot be used or would only serve to further exacerbate the situation. The availability of non-lethal weapons expands the range of options available to law enforcement in reacting to potentially violent or life-threatening situations.
Hand-held flashlights have been in widespread usage in many areas for many years. For example, in the area of forensics, law enforcement personnel have found flashlights to be so useful that incandescent flashlights have become xe2x80x9cstandard issue.xe2x80x9d These xe2x80x9cstandard issuexe2x80x9d flashlights produce a bright beam from a small and relatively light-weight package. Such xe2x80x9cstandard issuexe2x80x9d flashlights have, inter alia, been used by law enforcement personnel to illuminate crime scenes, to disorient and confuse suspects and to physically subdue suspects.
The effect of laser light on human eyesight can be separated into three categories: glare effects, dazzling or flashblinding, and permanent damage. The retinal damage threshold in general depends upon the laser wavelength, the exposure duration, and whether or not the laser is operated in continuous wave or pulsed or modulated mode and, if pulsed, the repetition rate and the pulse duration. If the laser light intensity is such that the damage threshold for the eye is exceeded, lesions are produced that are permanent. This damage occurs at the location of the retina for visible and near-infrared light. Ultraviolet and far-infrared light on the other hand are absorbed in the cornea and the light never reaches the retina.
The exact intensities where glare stops and dazzling occurs are difficult to define with precision and depend to a large extent on the individual involved. In general, however, glare occurs first and results in little or no loss of vision performance. In fact, after a laser source is turned off, in the glare region no after-effects or latent images occur. If the intensity is increased, however, at some intensity or over a range of intensities, dazzling occurs. If the laser is turned off, short-term partial loss of vision occurs, typically lasting for seconds or tens of seconds. At still greater intensities the after-effect is substantially longer, perhaps as long as-minutes. This effect is best compared to the use of a fundus camera to photograph the retina of the human eye, usually under fully dilated conditions. Substantial vision loss can occur and last for many minutes.
Some inroads have also been made in applying laser technology to portable illumination devices in limited areas. One major drawback of such uses of lasers has been that the laser beam emitted by such low-power devices has the potential to produce irreversible eye damage if a person gazes directly into the light source. Thus, these devices are not xe2x80x9ceyesafe.xe2x80x9d Naturally, this problem becomes exacerbated as attempts are made to increase the output power of such devices. Another significant limitation associated with such portable laser emitting devices is that they have, to date, been unable to produce nearly as much light as comparably sized incandescent flashlights. Accordingly, their lack of versatility and overall poor performance has limited their use.
The desirability of producing glare or flashblind effects, whereby a temporary reduction to visual performance results from exposure to laser light, has been disclosed, among others, by German in U.S. Pat. No. 5,685,636. However, the laser flashlight device as described by German suffers from a number of critical deficiencies, especially with regard to the laser safety aspects associated with its intended use as a portable visual security device against mobile targets. For example, the eye safety of the radiation produced with the portable laser security device of German, could only be assured beyond a certain range, which, at the minimum, is set at 3 m. Thus, to keep the intensity below 58.3 mW/cm2, the upper limit recited as corresponding to permanent undesirable damage to the eye, would require spot size diameters of at least 5 cm for power levels beyond 1 W. Since the damage threshold decreases as the exposure time increases (FIG. 1 of U.S. Pat. No. 5,685,636) the operational range of power must be reduced and/or the beam spread parameter increased to assure safety at exposure times which may be longer than 100 msec.
German teaches that the spot size is to be adjusted using a movable collimating lens contained within the apparatus, with the explicit purpose of reducing the laser beam spread produced by a semiconductor laser with a highly divergent beam. Clearly, such manual adjustments may be difficult to realize in real life situations where rapidly moving intruders are encountered. It will require the operator to guess the likely range of the target while taking additional care to ascertain that the range always exceed a minimum value even as the exposure duration is kept short enough to avoid permanent eye damage, yet long enough to produce the desired deterrent effect. These are clearly difficult conditions to fulfil in high stress situations where rapid response times are essential.
In terms of the effectiveness of the laser flashlight in use as a non-lethal security device, the embodiments and methods of operation as taught by German again fall short. In particular, whereas it was appreciated that shorter wavelengths are more effective in producing the desired glare and flashblind effects, requiring less power than a red semiconductor laser, no disclosure was provided with regard to either methods and/or structures for producing said shorter wavelengths laser devices, in particular, at the green wavelengths recognized as especially effective for this application.
It may be noted that the specific example provided, namely the laser produced by Santa Fe lasers, is neither compact enough to be provided within a standard flashlight package, nor is it capable of producing the desired eye safety features. In particular, it must further be noted that with regard to the green radiation produced by frequency doubled, diode pumped solid state laser, it is well known in the art that the beam properties from such a laser are very different than those produced by typical diode lasers. For example, the laser radiation of a solid state laser tends to be much less divergent, having a higher degree of spatial coherence than the output of a diode laser. Consequently the collimating lens described by German as the critical element for reducing the spread of a laser beam is entirely incompatible with the near diffraction limited radiation produced from most crystalline solid state laser. In fact, use of such a short focal length lens may result in drastic focusing effects leading to smaller beam spot sizes and therefore much higher intensities even at the longest ranges cited by German, thereby severely compromising eye safety. It was clearly not realized by German that use of a movable collimating or focusing lens as a means for adjusting the beam spread of the laser device is insufficient to assure eye safety at arbitrary ranges from the solid state laser device. Although other optical elements may be envisioned that may be capable of increasing rather than decreasing the beam spread, no such elements were described in the German patent, yet the choice of a specific such optical element represents an essential design feature for the security applications contemplated.
Alternative prior art that directed to portable laser applications also fail to fulfil the requirements of a safe and effective laser flashlight. For example, U.S. Pat. No. 5,396,069 to Craig et al describes a packaged solid state laser device for use as a portable night vision apparatus. The laser disclosed therein does not include beam shaping or other optics necessary to assure eye safety at all power levels. In particular Craig teaches that his device specifically requires eye safety warning labels, and that such device is recognized not to be eye safe when used with the recommended light source, in this case a diode laser of power up to 50 mW. Craig also teaches a variable focus adjustment system, which is at the operator""s disposal. Clearly, such adjustments will be difficult to execute manually and still provide an output intensity that is an effective deterrent against a moving target, while also assuring safety according to accepted ANSI standard for a given exposure duration. Furthermore, none of the embodiments shown conform to a standard flashlight packaging constraints imposed herein. Other compact solid state laser have been described in the literature, but none that would have the necessary eye safety features at sufficiently high power levels to provide only reversible flashblinding or glare effects.
It should be readily appreciated that to guarantee eye safety under all conditions, a laser flashlight used as a security device against moving human targets must be eye safe at the aperture, or right at the exit face of the flashlight device. Only then can the operator be assured that the laser radiation would not produce permanent, lethal eye damage at any range, for a given set of power settings, selected to provide the desired effects. Yet, without such assurance, the likelihood of use of said flashlight by law enforcement personnel is not very high. None of the laser technologies or commercially available devices known in the current art include the requisite fail safe mechanism for assuring both automatic safety and effectiveness.
Briefly stated, the invention in a preferred form is a laser flashlight for emitting an eyesafe light dispersion pattern into the eyes of a person. The laser flashlight employs an emitter disposed within the flashlight housing for emitting a coherent light beam along an optical axis toward the exit face of the housing. An optical system, disposed along the optical axis intermediate the emitter and the exit face, is configured to provide an output laser beam having a spatial profile that is substantially gaussian. A light transmissive beam expander, disposed along the optical axis proximate to the exit face of the housing, is configured to limit the intensity of the output laser beam at the eyes of the person, for any power of the emitter, up to and including the maximum power of the emitter, to a level which is always less than the ANSI safety standard adapted for the wavelength of the output laser beam.
In different embodiments of the laser flashlight of the present invention, the emitter and the optical path configuration can be either a diode laser optically connected to an optical fiber or, alternatively, a diode laser or laser array pumping a solid state crystal disposed within an optical resonator. The optical system may also include one or more lenses to focus the coherent light emitted by the emitter on a laser crystal. Thus, the emitter can be directly coupled to the beam expander via an optical fiber or coupled therewith using a resonator. Further, the laser beam emitted by the flashlight may be either continuous wave or pulsed.
The laser resonator comprising the optical system of the flashlight may further include a harmonic generating crystal to shift the wavelength of the laser beam to either a longer or shorter spectral range. In alternative embodiments, the harmonic generating crystal may be a nonlinear or a linear wavelength shifting element. Preferably, and optimally when the flashlight is used as a security device, the harmonically generated light has a wavelength in the visible range, close to the green portion of the spectrum where light sensitivity of the eye is at a maximum. In such a laser flashlight, the optical system preferably includes optical coatings designed to reverse the direction of the harmonic light which is traveling back toward the emitter to thereby increase the efficiency of the flashlight.
It is, accordingly, an object of the present invention to provide an eyesafe laser flashlight which will not produce irreversible eye damage if a person gazes directly into the flashlight.
It is a further object of the present invention to provide a portable laser flashlight having the general size, shape and weight characteristics of a xe2x80x9cstandard issuexe2x80x9d incandescent flashlight used by law enforcement personnel.
It is still another object of the present invention to provide a hand-held laser flashlight which is capable of emitting significantly more light than prior hand-held laser-based illumination devices to thereby enable the laser flashlight to be effectively used over longer distances and/or to illuminate larger areas.
It is yet another object of the present invention to provide a portable laser flashlight having at least one laser emitter that can be modulated in either periodic or random fashion to more efficiently produce a bright beam of light.
It is an additional object of the present invention to provide an eyesafe portable laser flashlight which employs at least one laser emitter and is capable of operating at more than one output wavelength.
It is still another object of the present invention to provide a hand-held laser flashlight which provides an optimal combination of (a) simplicity; (b) reliability; (c) durability; (d) versatility; and (e) efficiency.
Numerous other advantages and features of the present invention will become apparent to those of ordinary skill in the art from the following detailed description of the invention, from the claims and from the accompanying drawings.
It is still another object to of the present invention to provide a portable laser flashlight including the feature of constant brightness over a range of power settings. This feature enables variation of the power settings without affecting the beam spread, thereby assuring eye safety even at the highest power settings.